1. Field of the Invention
This invention relates to a process for producing ceramic substrates having metals joined thereto. More particularly, the invention relates to a process for producing aluminum or aluminum alloy-ceramics composite electronic circuit boards suitable for mounting highpower electronic devices such as power modules.
This invention further concerns a process for producing rugged metal-bonded-ceramic (MBC) material or components. Particularly the present invention is directed to an industrial process for producing MBC material or components of metals with oxide, nitride or carbide ceramics that are suitable for use in automotive parts, electronic parts, etc. The invention also is directed to MBC plates produced by such process, as well as electronic circuit substrates fabricated from such MBC plates.
2. Background Information
Metal-ceramics composite electronic circuit boards which have metals joined to the surface of ceramic substrates are commonly used for mounting high-power devices such as power modules. To make such composite circuit boards, copper, which has high electrical and thermal conductivities, is joined to alumina or aluminum nitride ceramics.
MBC components are extensively used in automobiles, electronic equipment and other applications by taking advantage of the chemical stability, high melting points, insulating property and high hardness of ceramics in combination with the high strength, high toughness, free workability and conducting property of metals. Typical applications of the components include rotors for automotive turbochargers, as well as substrates and packages for mounting large-power electronic elements.
MBC material and MBC components are known to be produced by various methods including adhesion, plating, metallizing, thermal spraying, brazing, direct bonding or "DBC", shrink fitting and casting.
Adhesion is a process in which a metal member is joined to a ceramic member with an organic or inorganic adhesive.
Plating is a process that comprises activating the surface of a ceramic member and dipping it in a plating bath to apply a metal plate.
Metallizing is a process that comprises applying a paste containing metal particles to the surface or a ceramic member and sintering it to form a metal layer.
Thermal spraying is a process in which molten metal (or ceramic) drops are sprayed onto the surface of a ceramic (or metal) member so that a metal (or ceramic) layer is formed on that surface.
The direct bonding process (DBC) has been developed for the particular purpose of joining copper to oxide ceramics. In the DBC method, an oxygen-containing copper member sheet is heated in an inert gas atmosphere while it is joined to a ceramic member or, alternatively, oxygen-free copper is heated in an oxidizing atmosphere, to join copper to a ceramic member, i.e., the surface of an oxygen-free copper member is first oxidized to form a copper layer then joined to a ceramic member. Thus in order to join copper to non-oxide ceramics by the DBC method, an oxide layer must first be formed on the surface of a non-oxide ceramic substrate. As disclosed in Unexamined Published Japanese Patent Application (Kokai) No. 3077/1984, an aluminum nitride substrate is first treated in air at a temperature of about 1000.degree. C. to form an oxide on the face of the substrate and the direct bonding method is then applied to join copper to the aluminum nitride.
In brazing, copper is joined to ceramics with the use of an intermediary brazing material containing an active metal. The most common brazing material used in this method is based on a Ag--Cu--Ti system.
Brazing is a process in which metal and a ceramic member are joined with the aid of a low-melting point filler metal or alloy. To insure that the filler metal or alloy is securely joined to the ceramic member, a metal component highly reactive with ceramics is added or a metal layer is preliminarily formed on the joining surface of the ceramic member by a suitable method such as metallizing or thermal spraying.
In shrink fitting, the ceramic and metal members to be joined are provided with a projection and a cavity, respectively, in such a way that the outside diameter of the projection is equal to the inside diameter of the cavity and the metal member is heated to expand the cavity, into which the projection of the ceramic member is inserted; thereafter, the two members are cooled so that they form an integral assembly in which the projection on the ceramic member is nested in the cavity in the metal member.
Casting is similar to shrink fitting, except that a metal is cast around a ceramic member and cooled so that it shrinks to have the ceramic member nested as an integral part.
These prior art methods, however, have problems. Adhesion produces composites that are low in adhesion strength and heat resistance. The applicability of plating, metallizing and thermal spraying is usually limited to the case of forming thin metal (or ceramic) layers whose thickness ranges from a few microns to several tens of microns.
Shrink fitting and casting are applicable only to a special case in which at least part of a ceramic member is to be nested in a metal.
In DBC, copper is the only metal that can be joined and the temperature for joining must be within a narrow range close to the eutectic point of Cu--O and, hence, there is a high likelihood for the development of joining defects such as swelling and incomplete joining.
Brazing uses expensive filler metals or alloys and requires the joining operation to be performed in a vacuum and, hence, the operational cost is high enough to prevent the use of the method in a broad range of applications.
In spite of their widespread use, the copper-ceramics composite substrates have several problems associated with production and practical use. The most serious problem is that cracks can develop in the ceramic substrates during mounting electronic components and during their subsequent use, whereupon dielectric breakdown may occur across the thickness of the substrate.
In order to join copper to ceramic substrates, they are heated to almost 1,000.degree. C. and, in addition, the copper-ceramics composite substrate is heated to almost 400.degree. C. when mounting electronic components such as power modules. The thermal expansion coefficient of copper is higher than that of ceramics by a factor of about 10, so when the composite substrate is cooled to room temperature, the thermal expansion mismatch will cause a substantial thermal stress within the substrate.
Furthermore, on account of the environment in which the electronic components are used, as well as the heat generated during their use, the temperature of the composite substrate is subject to constant changes, which cause corresponding changes in the thermal stress on the substrate. Such thermal stresses contribute to cracking in the ceramic substrate. One of the important parameters for evaluation of ceramic electronic circuit boards is resistance to heat cycles, or the number of thermal swings between -40.degree. C. and 125.degree. C. that can be applied to the circuit board until cracking occurs. Copper-alumina composite substrates fabricated by the direct bonding method can withstand 20 such heat cycles but, on the other hand, similar substrates fabricated by brazing can withstand only 10 heat cycles or less.
Aluminum has comparable electrical and thermal conductivities to copper and the idea of using aluminum as a conductive circuit material has been known for many years (see, for example, Unexamined Published Japanese Patent Application (kokai) No. 121890/1984, which discusses such idea). Aluminum is softer than copper and its yield strength is about a quarter of the value for copper. Therefore, it is anticipated that the residual stress in the composite substrates could be markedly reduced by using aluminum as a circuit material. However, Unexamined Publication Japanese Patent Application (kokai) No. 121890/1984, supra, do not disclose a specific method of joining aluminum to ceramics.
Unexamined Published Japanese Patent Application (kokai) Nos. 125463/1991, 12554/1992 and 18746/1992 disclose methods of fabricating aluminum-ceramics composite substrates by brazing. According to these published Japanese patent documents, the fabricated aluminum-ceramics composite substrates can withstand at least 200 heat cycles, which is about 10 times as great as the value for the copper-ceramics composite substrate.
However, the aluminum-ceramics composite substrates that are produced by the methods disclosed in the published Japanese patent documents listed above have the following problems associated with their manufacture or subsequent use:
(1) Aluminum is highly prone to be oxidized, so the methods at issue have to be carried out in either in vacuo or in an atmosphere filled with an inert gas of high purity.
(2) The melting point of aluminum is as low as 660.degree. C., so if the brazing temperature approaches this level, aluminum will either totally melt to lose its shape or partly melt to induce a brazing defect called "nibbling". On the other hand, if the brazing temperature is unduly low, the brazing material will not completely react with the ceramics and the resulting composite will have only a small strength. As will be described hereinabove in the Examples, the use of a brazing alloy of an Al--Si system requires that brazing be carried out within a temperature range even narrower than the range from 590.degree. C. to 640.degree. C. According to the experience of the present inventors, controlling the temperature in a heating furnace to reach uniformity is very difficult to accomplish in large-scale production, particularly in the case where composite substrates are to be produced in vacuo (because the heat from a heating element is transmitted solely by radiation and conduction in the absence of convection).
(3) When composite substrates are to be fabricated by brazing, the brazing temperature must not be higher than the melting point of aluminum (660.degree. C.). However, the aluminum-based brazing material wets the ceramics only poorly at such low temperatures. Thus, failures in joining often occur if composite substrates are produced by brazing.
(4) An alumina substrate is amenable to direct brazing. However, an aluminum nitride ceramic substrate requires that its surface be oxidized as in the case of direct joining of copper. However, this is not desirable for the following two reasons: first, the overall process becomes complicated; second, the aluminum nitride substrate has been developed in order to meet the requirement for better thermal conductivity, but an oxide formed on the surface is, of course, detrimental to the thermal conduction characteristics.
(5) As in the case of copper-ceramics composite substrates, the aluminum-ceramics composite substrates fabricated by brazing would have a lower resistance to heat cycles than those fabricated by direct bonding. This is because the brazing alloys used in the fabrication of the substrates are harder than pure metals and will not readily undergo plastic deformation, thus causing untoward effects on stress relaxation.
As described above, the process of fabricating aluminum-ceramics composite substrates by brazing has several problems including instability. If the ceramic substrate is based on a non-oxide, the manufacturing process is complicated. Furthermore, the composite substrates fabricated by the prior art processes are not always considered to be optimal with respect of joint strength and heat resistance characteristics.
As already hereinabove, MBC components can be used as substrates for mounting large-power electronic elements. Currently used MBC substrates comprise ceramic substrates having copper circuit patterns formed on the surface. Depending on the kind of the ceramic substrate used and the process for producing the MBC substrates, commercial composite substrates are classified as copper/alumina directly joined substrates, copper/aluminum nitride directly joined substrates, copper/alumina brazed substrates and copper/aluminum nitride brazed substrates.
A method for producing copper/alumina directly joined substrates is described in Unexamined Published Japanese Patent Application (kokai) Sho-52-37914. In Sho-52-37914, an oxygen-containing copper plate is superimposed on an alumina substrate or, alternatively, an oxygen-free copper plate is heated in an oxidizing atmosphere to generate copper oxide on the surface of the oxygen-free copper plate, before the copper plate is joined to the alumina substrate with a composite oxide of copper and aluminum being formed at the interface.
To produce copper/aluminum nitride directly joined substrates, an oxide must first be formed on the surface of an aluminum nitride substrate. A method of production is described in Unexamined Published Japanese Patent Application (kokai) Hei 3-93687. In Hei 3-93687 the aluminum nitride substrate is first treated in air at a temperature of about 1,000.degree. C. to form an oxide layer on the surface and, then, a copper plate is joined to the aluminum nitride substrate with an intermediate oxide layer, by the method described in the preceding paragraph.
To produce copper/alumina or aluminum nitride brazed substrates, a copper plate is joined to a ceramic substrate with the aid of a low-melting point filler metal. A commonly used filler metal is made from an alloy comprising Ag, Cu and Ti. In this case the purpose or adding Ag is to lower the melting point and the purpose of adding Ti is to enhance the wettability of the filler metal to ceramics.
In spite of their common use, copper/ceramic composite substrates have several problems that are encountered during fabrication and actual use. The most serious problem is that cracks develop in the ceramic substrate during the mounting of electronic element and during the use of thus assembled electronic devices. This defect is attributable to the thermal expansion coefficient of copper, which is higher than that of ceramics by about one order of magnitude.
During the joining operation, both the ceramic substrate and copper are heated to about 1,000.degree. C. and as they are cooled from the joining temperature to room temperature, considerable thermal stress therefore develops within the MBC substrate due to the thermal expansion difference. When mounting electronic elements on MBC substrates, the latter are heated to about 400.degree. C. and also the temperature of the MBC substrate constantly change due to the temperature change in the operating environment or due to the generation of heat during the use of the assembled electronic devices, whereby the MBC substrates are subjected to varying thermal stresses. On account of these thermal stresses, cracks develop in the ceramic substrate.
An important factor regarding the evaluation of the MBC substrates is resistance to heat cycles. This factor is expressed by the number of times a substrate can be subjected to repeated heating and cooling cycles between -40.degree. C. and 125.degree. C. without developing cracks due to thermal stresses. The copper-bonded-ceramic substrates can withstand only several tens of heat cycles under the indicated conditions. Moreover, in order to acquire resistance to the heat cycles, the thickness of the ceramic substrate has to be greater than the sum of the thicknesses of copper plates joined to both principal surfaces of the ceramic substrate. This means that the thickness of the ceramic substrate must be increased beyond the value necessary to maintain its inherent electrical insulating property. As a result, heat conduction, which is another important characteristic of the MBC substrates, is sacrificed.
In recent years, the development of power modules for installation on electric cars has increased the demand for MBC substrates having improved resistance to heat cycles. Under severe use conditions as in electric cars where temperatures changes are violent and great vibrations occur, resistance to 3,000 heat cycles and more is said to be necessary. However, this requirement cannot be met by the copper-bonded-ceramic substrates in current use.
Aluminum is as good an electric and heat conductor as copper and the idea of using it as a conductive circuit material is described in Unexamined Published Japanese Patent Application (kokai) Sho 59-121890. Generally brazing is used to join aluminum and ceramics and Unexamined Published Japanese Patent Applications (kokai) Hei 3-125463, Hei 4-12554 and Hei 4-18746 teach aluminum-ceramic substrates produced by brazing. These substrates can withstand about 200 heat cycles, but they are still unsuitable for use in electric cars and in other applications where very high resistance to heat cycles is required. As a further problem, brazing must be conducted in a vacuum and nonoxide ceramics must be subjected to a preliminary treatment for forming an oxide on the surface and this makes the brazing process unsatisfactory in terms of not only production cost, but also heat conduction.
Thus, the prior techniques for joining metal and ceramic members have had one or more of the following problems:
(1) the metal and ceramic members to be joined are limited to those having certain shapes;
(2) the joining step is costly and limited in the scope of applications; and
(3) the joined members do not meet the performance requirements for joint strength, heat resistance and resistance to heat cycles.